Besides endoscopes for medical and non-medical technical applications, the viewing direction of which is parallel to the longitudinal axis of the shaft of the endoscope, endoscopes having different fixed viewing directions were developed early on. Here and in what follows, the viewing direction of an endoscope always means the direction from the distal end of the endoscope in which an object, which appears in the middle of the image acquired by means of the endoscope, lies. In many applications, however, a fixed viewing direction is disadvantageous. In the least favorable case, for example, the endoscope must be changed several times during a medical intervention. In such cases, the use of an endoscope having an in-situ adjustable or variable viewing direction is advantageous.
The observation of an object in a cavity by means of an endoscope generally requires illumination of the object. To this end, for example, an endoscope comprises light waveguides, in particular glass fibers, by means of which illumination light is transmitted from the proximal end of the endoscope along the shaft to the distal end of the endoscope. Light exit surfaces of the light waveguides at the distal end of the endoscope are arranged and formed in such a way that the entire field of view, or viewing field, is illuminated sufficiently.
In an endoscope having an adjustable viewing direction, in the simplest case the illumination light is distributed at the distal end of the endoscope in such a way that the entire field of view is illuminated independently of the respectively adjusted viewing direction. This, however, entails a number of disadvantages. In particular, light power is wasted, because the entire fields of view of all adjustable illumination directions are constantly illuminated, independently of the actually adjusted viewing direction. For a predetermined desired brightness, a significantly higher light power must therefore be provided overall than in an endoscope having a fixed viewing direction.
Another disadvantage is based on the fact that high-intensity illumination light can damage tissue or other objects photothermally or photochemically. In an endoscope having a fixed viewing direction, too small a distance of the distal end of the endoscope from an object is generally apparent, at least when observing the acquired image. When using a camera on the endoscope, automatic warning of users is also possible when the brightness of an acquired image exceeds a predetermined threshold. In an endoscope having an adjustable viewing direction, however, a part of the illumination light strikes objects which lie outside the field of view. Undesired proximity of the distal end of the endoscope to these objects, and resulting exposure of these objects to a radiation power which is too high, are therefore not apparent.
Another disadvantage is that illumination light which is emitted outside the field of view can also be scattered or reflected by objects or opaque media. The reflected or scattered illumination light may directly or indirectly enter the observation beam path. Contrasts and, above all, the distinguishability of objects in dark image regions can thereby be reduced.
Another disadvantage is due to the fact that the illumination strength or intensity of the illumination light is essentially constant in the direction in which the viewing direction can be varied (also referred to as the vertical direction), while in the direction perpendicular thereto (also referred to as the horizontal direction) it generally decreases slightly toward the edge of the field of view. From endoscopes having a fixed viewing direction, however, users are generally used to an illumination strength which decreases slightly toward the edge of the field of view both in the horizontal direction and in the vertical direction. The constant illumination strength in the vertical direction can therefore be perceived as irritating.
DE 600 15 375 T2 describes an arrangement of a plurality of prisms. One of the prisms can be rotated about an axis in order to shine illumination light in an adjustable viewing direction. The inventors of the present invention have, however, established that the mounting of the rotatable prism is in many cases elaborate in terms of design and production.